fix(log): avoid panic when showing log of a branch with no commits

[StressTestor]

Problem

Opening the log of a branch with no commits panics (fixes #262):

mkdir example && cd example && git init
gitu      # press `l l` to show the log of the current branch

panicked at src/screen/mod.rs:99: index out of bounds: the len is 0 but the index is 0

Root cause

The log screen for a branch with no commits has no items, so line_index is empty. But Screen::new → update() → update_cursor() still tries to position the cursor: move_from_unselectable calls nav_filter(self.cursor) → at_line(0), which does &self.items[self.line_index[0]] and panics on the empty line_index.

Fix

Guard update_cursor to reset the cursor when nothing is selectable, instead of positioning it into an empty line_index. This matches the existing empty-checks the other cursor/scroll helpers already use (scroll_fit_start, scroll_fit_end, move_cursor_to_screen_line).

I kept the fix to the single chokepoint on the panic path rather than changing get_selected_item() / at_line() to return Option, since get_selected_item() has ~12 call sites across the crate — that would be a large ripple for this bug.

Test

Added log_empty_branch in src/tests/log.rs (same empty-repo setup as the existing fresh_init test, then l l). Before the fix it panics; after, it renders the empty log. The snapshot is blank because a branch with no commits has nothing to show — the test's purpose is that a regression would panic before reaching the assertion.

make test passes locally (cargo insta test --unreferenced reject, cargo clippy -- -Dwarnings, cargo fmt --check, cargo bench --no-run); full suite is green.

[altsem]

nice!